In general, phenomena affecting the surface condition of a metal component in an installation containing an ionic phase (for example an electrolytic liquid) can be influenced by various working conditions, amongst which there may be mentioned, in particular, the nature of the material constituting the component in question, the shape, the dimensions and the surface condition of the latter (in particular its roughness), the characteristics of the ionic phase treated in the installation (for example the value of the pH), the pressure or temperature conditions in the installation, the possible presence of a galvanic couple between two different metals or alloys, a local deterioration of a film for protecting a wall of the installation (for example a deterioration of the layer of zinc on a galvanized steel pipe), the appearance of a localized air pocket in a dead zone of the installation, the presence of an electrokinetic phenomenon in a constricted zone of piping in which the ionic phase circulates, the existence of anodic or cathodic protection for the installation, and the like. Furthermore, the phenomena affecting the surface condition of the metal component in the installation can be of a diverse nature and can consist, in particular, of corrosion, erosion or encrustation.
Thus, in the presence of hard water, there is generally a gradual scaling-up of the metal installations; this scaling-up,which is particularly significant in the case of hot water, leads to a gradual clogging of the installations and to a drop in the efficiency of the heat exchangers.
In the presence of chemically aggressive water, such as softened water, stainless steel or galvanized steel installations run the risk of suffering local corrosion which can sometimes lead to a hole in the wall of the installation.
If the liquids treated contain suspended solids, local erosion of the walls of the installation, or sedimentation in those zones of the installation in which the liquid undergoes a sudden pressure loss, such as the widenings in piping or the elbows, is sometimes observed.
Erosion or sedimentation phenomena can occur, in particular, in installations through which viscous liquids, such as sludges, pass, or in evaporator/crystallizers such as those commonly used for the treatment of aqueous solutions of caustic soda originating from cells for the electrolysis of sodium chloride brine.
In general, it is important to be able to determine with precision the influence that these various working conditions can exert, in isolation or in combination, on the change in the surface condition of a given metal component in an installation containing an ionic phase.
Furthermore, during the operation of an industrial installation, it is important to be able to monitor, at any time, the change in the surface condition of the metal components constituting the installation, so that the appearance of a serious anomaly in this surface condition can be detected rapidly and so that it is then possible to react immediately and alter the working conditions in the installation in order to rectify this anomaly.
It is known to use methods of gravimetric measurement for controlling the corrosive or encrusting nature of liquids circulating in installations (Materials Protection, October 1962, pages 10 to 19 and 27). These known methods consist in periodically extracting, from the liquid, a probe which is normally immersed therein, in scraping from the probe the materials which may have become encrusted thereon, and in weighing these materials and also the probe. A comparison of the weight of the probe before and after the test makes it possible to assess the corrosive nature of the liquid, whilst the weight of the materials which have become encrusted on the probe during the test is a measure of the encrusting nature of the liquid. These known methods exhibit the disadvantage of being slow and relatively imprecise, and they are incapable of providing an instantaneous indication of the surface condition of an installation in which the liquid is being treated.
It has also been proposed to monitor the corrosive nature of liquids circulating in metal installations by measuring the variation, with time, of the electrical resistance of a probe immersed in the liquid (Corrosion, National Association of Corrosion Engineers, Volume 14, March 1958, pages 155t to 158t). Although it permits precise monitoring of the aggressiveness of the liquid, this known process does not make it possible to evaluate the rate of degradation of the material constituting the installation.
In order to overcome this disadvantage, the technique referred to as the "slope at the origin" method (also known as the "polarization resistance" method) has been proposed; this technique consists in immersing, in the liquid, a probe made of an identical material to that of the installation examined, and in determining the current/voltage ration in the region of the equilibrium potential of the probe. This technique makes it possible to evaluate the change in the surface condition of the probe, but it involves the use of expensive precalibrated probes which should be replaced periodically. This known technique exhibits the additional disadvantage that it does not generally permit instantaneous and continuous measurements.
It has also been proposed to monitor the formation of deposits or encrustations on a wall in contact with a liquid, for example the wall of a heat exchanger, by measuring the variation, with time, of the temperature of the wall by means of a thermocouple housed in the wall (Chemical Engineering Progress July 1975, Volume 71, No. 7, pages 66 to 72). However, this known method is not suitable for monitoring the corrosive nature of the liquid. It also exhibits the disadvantage of being highly dependent on the temperature variations in the medium in contact with the wall.
U.S. Pat. No. 3,612,998 proposes a process for detecting corrosion caused by an electrokinetic phenomenon created by the travel of a liquid at high speed in the vicinity of a metal component, the process consisting in measuring the electric current generated by the continuous dissolution of the metal component in the liquid, under the effect of the electrokinetic phenomenon.
This known process exhibits the disadvantage that it only applies to a particular type of corrosion, namely that caused by the flow of liquids at very high speed. It is not capable of detecting other types of corrosion, such as, for example, that which is inherent in the aggressiveness of soft waters, or of detecting erosion or encrustation. It exhibits the additional disadvantage that it does not represent the surface condition of the metal component and, consequently, it does not make it possible to assess the type of corrosion suffered by the metal component.
The known processes described above all exhibit the additional disadvantage that they are incapable of detecting the cause which gives rise to the appearance of a phenomenon affecting the surface condition of a metal component in the installation. In other words, although they detect, generally with precision, the appearance of such a phenomenon, for example local corrosion, these known processes provide no indication as regards the inadvertent modifications which have occurred in the working conditions of the installation and which give rise to the appearance of this phenomenon. Similarly, these known processes do not make it possible to easily assess the influence which a given modification of the working conditions would exert on the surface condition of the metal components in the installation.